Germanium/silicon avalanche photodetector with separate absorption and multiplication regions

ABSTRACT

A semiconductor waveguide based optical receiver is disclosed. An apparatus according to aspects of the present invention includes an absorption region including a first type of semiconductor region proximate to a second type of semiconductor region. The first type of semiconductor is to absorb light in a first range of wavelengths and the second type of semiconductor to absorb light in a second range of wavelengths. A multiplication region is defined proximate to and separate from the absorption region. The multiplication region includes an intrinsic semiconductor region in which there is an electric field to multiply the electrons created in the absorption region.

REFERENCE TO PRIOR APPLICATION

This application is a continuation of and claims priority to U.S.application Ser. No. 11/170,556, filed Jun. 28, 2005, now U.S. Pat. No.7,233,051.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of invention relate generally to optical devices and, morespecifically but not exclusively relate to photodetectors.

2. Background Information

The need for fast and efficient optical-based technologies is increasingas Internet data traffic growth rate is overtaking voice traffic pushingthe need for fiber optical communications. Transmission of multipleoptical channels over the same fiber in the dense wavelength-divisionmultiplexing (DWDM) system provides a simple way to use theunprecedented capacity (signal bandwidth) offered by fiber optics.Commonly used optical components in the system include wavelengthdivision multiplexed (WDM) transmitters and receivers, optical filtersuch as diffraction gratings, thin-film filters, fiber Bragg gratings,arrayed-waveguide gratings, optical add/drop multiplexers, lasers,optical switches and photodetectors. Photodiodes may be used asphotodetectors to detect light by converting incident light into anelectrical signal. An electrical circuit may be coupled to thephotodetector to receive the electrical signal representing the incidentlight. The electrical circuit may then process the electrical signal inaccordance with the desired application.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1A is a diagram illustrating a cross-section view of a plurality ofgermanium/silicon avalanche photodetectors with separate absorption andmultiplication regions in a system for an embodiment of the presentinvention.

FIG. 1B is a diagram illustrating a top view of a plurality ofgermanium/silicon avalanche photodetectors with separate absorption andmultiplication regions arranged in a two-dimensional array for anembodiment of the present invention.

FIG. 2 is a diagram illustrating responsivity versus wavelengthrelationships with respect to the silicon and germanium layers of anabsorption region of an avalanche photodetector for an embodiment of thepresent invention.

FIG. 3 is a diagram illustrating an improvement in sensitivity with theuse of silicon in the multiplication region of a germanium/siliconavalanche photodetector with separate absorption and multiplicationregions for an embodiment of the present invention.

FIG. 4A is a diagram illustrating a cross-section view of agermanium/silicon avalanche photodetector with a resonant cavity for anembodiment of the present invention.

FIG. 4B is another diagram illustrating a cross-section view of agermanium/silicon avalanche photodetector with a resonant cavity thatshows electron-hole pairs being generated for an embodiment of thepresent invention.

DETAILED DESCRIPTION

Methods and apparatuses for germanium/silicon avalanche photodetectors(APDs) with separate absorption and multiplication (SAM) regions aredisclosed. In the following description numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one having ordinary skill inthe art that the specific detail need not be employed to practice thepresent invention. In other instances, well-known materials or methodshave not been described in detail in order to avoid obscuring thepresent invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments. In addition, it is appreciated that the figures providedherewith are for explanation purposes to persons ordinarily skilled inthe art and that the drawings are not necessarily drawn to scale.

FIG. 1A is a diagram illustrating a cross-section view of a system 100including plurality of avalanche photodetectors 103A, 103B, . . . 103Narranged in a grid or an array 101 having one or more dimensions for anembodiment of the present invention. Illumination 117 is incident uponone or more of the plurality of avalanche photodetectors 103A, 103B, . .. 103N of the array 101. In the illustrated example, an image of anobject 116 may be focused onto the array 101 through an optical element130 with illumination 117. Thus, array 101 may function to sense images,similar to for example a complementary metal oxide semiconductor (CMOS)sensor array or the like.

To illustrate, FIG. 1B shows a top view of array 101 with the pluralityof avalanche photodetectors 103A, 103B, . . . 103N arranged in a twodimensional grid such that each of the plurality of avalanchephotodetectors 103A, 103B, . . . 103N function as pixels or the like foran embodiment of the present invention. The example illustrated in FIG.1B shows an image 118 of object 116 using the pixels of array 101 withinillumination 117.

It is noted that although FIGS. 1A and 1B illustrate an exampleapplication of the avalanche photodetectors being employed in a imagingsystem for explanation purposes, the avalanche photodetectors may beemployed in other types of applications in which for example thedetection of light having any of a variety of wavelengths includingvisible through infrared wavelengths is realized in accordance with theteachings of the presenting invention.

Referring back to FIG. 1A, optical element 131 may be a lens or othertype of refractive or diffractive optical element such that the image isfocused on array 101 with illumination 117. Illumination 117 may includevisible light, infrared light and/or a combination of wavelengths acrossthe visible through infrared spectrum for an embodiment of the presentinvention.

In the example illustrated in FIG. 1A, each of the plurality ofavalanche photodetectors 103A, 103B, . . . 103N includes semiconductormaterial layers, 105, 107, 109, 111, 113 and 115. A contact 131 iscoupled to layer 105 and a contact 133 is coupled to layer 115. For oneembodiment, layer 105 is a p+ doped layer of silicon having a dopingconcentration of for example 5e19 cm⁻³ and a thickness of for example100 nanometers. For one embodiment, layer 105 has a doping concentrationthat provides an improved electrical coupling between a contact 131 andlayer 105. For one embodiment, layers 107 and 109 are intrinsicsemiconductor material regions that form an absorption region 135 of theavalanche photodetector 103A. Layer 107 is a layer of intrinsic siliconand layer 109 is a layer of intrinsic germanium for one embodiment.Proximate to the absorption region 135 is a separate multiplicationregion 137, which includes a layer 113 of intrinsic semiconductormaterial such as silicon. As shown in the illustrated example, layer 113is disposed between a layer 111 of p− doped silicon and a layer 115 ofn+ doped silicon. For one embodiment, layer 111 has a thickness of forexample 100 nanometers and a doping concentration of for example 1-2e17cm⁻³. For one embodiment, layer 115 has a doping concentration of forexample 5e19 cm⁻³. In the illustrated example, each of the plurality ofavalanche photodetectors 103A, 103B, . . . 103N is coupled betweenground and a voltage V₁, V₂, . . . V_(n) such that each avalanchephotodetector is biased resulting in an electric field between layers105 and 115 as shown.

It is appreciated of course that the specific example dopingconcentrations, thicknesses and materials or the like that are describedin this disclosure are provided for explanation purposes and that otherdoping concentrations, thicknesses and materials or the like may also beutilized in accordance with the teachings of the present invention.

In operation, illumination 117 is incident upon layer 105 of one or moreof each of the plurality of avalanche photodetectors 103A, 103B, . . .103N. Layer 105 is relatively thin such that substantially all ofillumination 117 is propagated through layer 105 to layer 107 of theabsorption region 135. For one embodiment, the intrinsic silicon oflayer 107 absorbs the light having wavelengths in the range ofapproximately 420 nanometers to approximately ˜1100 nanometers. Most ofthe light having wavelengths greater than approximately ˜1100 nanometersis propagated through the intrinsic silicon layer 107 into the intrinsicgermanium layer 109 of the absorption region 135. The intrinsicgermanium of layer 109 absorbs that remaining light that propagatesthrough layer 107 up to wavelengths of approximately 1600 nanometers.

To illustrate, FIG. 2 is a diagram 201 that shows example responsivityversus wavelength relationships of silicon and germanium for anembodiment of the present invention. In particular, diagram 201 showsplot 207, which shows the responsivity of silicon with respect towavelength, and plot 209, which shows the responsivity of germanium withrespect to wavelength. For one embodiment, plot 207 may correspond tothe responsivity of the intrinsic silicon of layer 107 and plot 209 maycorrespond to the responsivity of the intrinsic germanium of FIG. 1A. Asshown in plot 207, the silicon absorbs light having wavelengths as shortas approximately 420 nanometers. As the wavelengths get longer, theresponsivity of silicon begins to drop off due to the lower absorptionof silicon at infrared wavelengths. Indeed, as the wavelength of lightincreases at this point, the silicon becomes increasingly transparent asthe light becomes more infrared. Thus, with respect to FIG. 1A, thelonger wavelengths of illumination 117 are not absorbed in layer 107 andare instead propagate through to layer 109. However, plot 209 shows thatthe germanium absorbs the longer wavelength light in layer 109 that ispropagated through layer 107 up to wavelengths of approximately 1600nanometers for an embodiment of the present invention. The silicon inlayer 107 absorbs the shorter wavelengths of light less thanapproximately ˜1000 nanometers, while at the same wavelength range thegermanium has a much larger absorption coefficient and would otherwisenot generate significant photocurrent due to surface recombination inaccordance with the teachings of the present invention.

Therefore, referring back to FIG. 1A, with the combination of theintrinsic silicon of layer 107 and the intrinsic germanium of layer 109in absorption region 135, illumination 117 is absorbed in the absorptionregions 135 of the avalanche photodetectors from visible light having awavelength of approximately 420 nanometers all the way up to longerinfrared wavelengths having wavelengths up to approximately 1600nanometers in accordance with the teachings of the present invention.This absorption of the light of illumination 117 in semiconductor layers107 and 109 results in the generation of photocarriers or electron-holepairs in the absorption region 135.

Due to the biasing and electric fields present in the avalanchephotodetector, the holes of the electron-hole pairs generated in theabsorption region 135 drift towards layer 105 and the electrons drifttowards layer 115. As the electrons drift into the multiplication region137, the electrons are subjected to a relatively high electric field inintrinsic silicon layer 113 resulting from the doping levels of theneighboring layers of p− doped silicon in layer 111 and n+ doped siliconin layer 115. As a result of the high electric field in layer 113,impact ionization occurs to the electrons that drift into themultiplication region 137 from the absorption region 135 in accordancewith the teachings of the present invention. Therefore, the photocurrentcreated from the absorption of illumination 117 in absorption region 135is multiplied or amplified in multiplication region 137 for anembodiment of the present invention. The photocarriers are thencollected at contacts 131 and 133. For instance holes may be collectedat contact 131 and electrons are collected at contact 133. Contacts 131and 133 may be coupled to electrical circuitry to process the signalspresent at each of the contacts 131 and 133 according to embodiments ofthe present invention.

As mentioned above, multiplication region 137 includes intrinsic siliconin layer 113 as will as silicon in neighboring p− doped and n+ dopedlayers 111 and 115, respectively. FIG. 3 is a diagram 301 illustratingan improvement in sensitivity that is realized for an embodiment of anavalanche photodetector utilizing silicon in the multiplication region137 instead of another material, such as for example indium phosphide(InP). In particular, diagram 301 shows a relationship between areceiver sensitivity dBm versus photomultiplication gain M for variousembodiments of an avalanche photodectector. In particular, plot 333shows a receiver sensitivity versus photomultiplication gainrelationship for an indium phosphide based avalanche photodetector whileplot 335 shows a receiver sensitivity versus photomultiplication gainrelationship for silicon based avalanche photodetector. As can beobserved in FIG. 3 by comparing plots 333 and 335, receiver sensitivityis improved by approximately 4-5 dB by using a silicon based avalanchedphotodetector instead of an indium phosphide based avalanchephotodetector for an embodiment of the present invention. This showsthat less power is therefore needed using silicon instead of indiumphosphide in multiplication region 137 to accurately detect a signalencoded in an optical signal received by an avalanche photodetector foran embodiment of the present invention.

The utilization of silicon in the multiplication region 137 for anembodiment of the present invention improves sensitivity of theavalanche photodetectors 103A, 103B, . . . 103N as shown in FIGS. 1A and1B because of the impact ionization properties of the electrons andholes in the material. For an embodiment of the present invention,substantially only one type of carrier, in particular electrons, areable to achieve impact ionization because of the use of silicon inmultiplication region 137. This can be seen quantitatively with thek-factor, which is the ratio of impact ionization coefficients of holesto electrons. Silicon has a k-factor about one order of magnitude lowerthan, for example, indium phosphide. A result of the use of silicon isthat substantially only electrons are selectively multiplied oramplified in multiplication region 137 instead of holes. Thus, noise andinstability in the avalanche photodetectors 103A, 103B, . . . 103N isreduced for an embodiment of the present invention compared to amaterial with a higher k-factor. An equation showing the excess noisetied to the k-factor (k) is:F _(A)(M)=kM+(1−k)(2−(1/M))  (Equation 1)where F_(A) is the excess noise factor and M is the gain of theavalanche photodetector.

The chances of runaway resulting from the generation more than one typeof carrier in multiplication region 137 is substantially reduced becausesubstantially only electrons are able to achieve impact ionization byusing silicon of multiplication region 137 for an embodiment of thepresent invention. To illustrate, the k-factor value of silicon for anembodiment of the present invention is less than 0.05 or approximately0.02-0.05. In comparison, the k-factor value for other materials such asfor example indium gallium arsenide (InGaAs) is approximately 0.5-0.7while the k-factor value for germanium is approximately 0.7-1.0. Thus,the k-factor value using silicon for an embodiment of the presentinvention is less than other materials. Therefore, using silicon for anembodiment of an avalanche photodetector in multiplication region 137results in improved sensitivity over avalanche photodetectors usingother materials such as indium gallium arsenide or germanium or thelike.

FIG. 4A is a diagram illustrating a cross-section view of agermanium/silicon avalanche photodetector 403 with a resonant cavity foran embodiment of the present invention. It is appreciated that avalanchephotodetector 403 shares similarities with the examples avalanchephotodetectors 103A, 103B, . . . 103N shown in FIGS. 1A and 1B and thatavalanche photodetector 403 may be used in place of any one or more ofthe avalanche photodetectors 103A, 103B, . . . 103N in accordance withthe teachings of the present invention. Referring back to the exampleshown in FIG. 4A, avalanche photodetector 403 includes layers, 405, 407,409, 411, 413 and 415. In the example illustrated in FIG. 4A, avalanchephotodetector 403 is disposed on a silicon-on-insulator (SOI) wafer, andtherefore, avalanche photodetector also includes a silicon substratelayer 419 and a reflective layer, which is illustrated in FIG. 4A as aburied oxide layer 425. For one embodiment, avalanche photodetector 403also includes guard rings 421, which are disposed at the surface andinto layer 407 on opposing sides of layer 405 at the surface of layer407 as shown in FIG. 4A.

For one embodiment, layer 405 and guard rings 421 are p+ doped siliconhaving a doping concentration that provides an improved electricalcoupling between a contact coupled to layer 405 and layer 407. For oneembodiment, guard rings 421 are disposed proximate to layer 405 as shownin FIG. 4A to help prevent or reduce electric field from extending to orpast the edges of avalanche photodetector 403. By helping to isolate orconfine the electric field within the structure of avalanchephotodetector 403, guard rings 431 help to reduce leakage current fromthe avalanche photodetector 403 structure in accordance with theteachings of the present invention.

For one embodiment, layers 407 and 409 form an absorption region 435 ofthe avalanche photodetector 403. Layer 407 is a layer of intrinsicsilicon and layer 409 is a layer of intrinsic germanium for oneembodiment. Proximate to the absorption region 435 is a separatemultiplication region 437, which includes a layer 413 of intrinsicsilicon. As shown in the depicted example, layer 413 is disposed betweena layer 411 of p− doped silicon and a layer 415 of n+ doped silicon. Forone embodiment, layers 411 and 415 having doping concentrations thatresult in a high electric field in layer 413 of multiplication region437. For example, layer 411 has doping concentration of for example1-2e17 cm⁻³ and layer 415 has a doping concentration of for example 5e19cm⁻³ for one embodiment. In addition, a lower electric field is alsopresent between layer 405 and layer 415 for an embodiment of the presentinvention.

In operation, as shown in FIG. 4A, illumination 417 is directed toavalanche photodetector 403 and is incident upon a surface of avalanchephotodetector 403. In the example illustrated in FIG. 4A, illumination417 is directed through free space and is incident upon a surface oflayer 405. The light from illumination 417 is absorbed in absorptionregion 435 and electrons from the photocurrent or electron-hole pairsgenerated in absorption region 435 are multiplied in multiplicationregion 437 as a result of impact ionization in accordance with theteachings of the present invention. For one embodiment, a resonantcavity is also defined in avalanche photodetector 403 between buriedoxide layer 425 and the surface of avalanche photodetector 403 on whichthe light of illumination 417 is incident. As a result, the lightillumination 417 circulates in the resonant cavity between buried oxidelayer 425 and the surface of the avalanche photodetector as shown inFIG. 4A as shown.

FIG. 4B is another diagram illustrating increased detail of across-section view of avalanche photodetector 403 with a resonant cavitythat shows electron-hole pairs being generated for an embodiment of thepresent invention. In particular, FIG. 4B shows illumination 417incident on the surface of layer 405 of avalanche photodetector 403. Asillumination propagates through layers 407 and 409 of the absorptionregion 435, the light is absorbed, which generates photocurrent orelectron-hole pairs including electron 427 and hole 429. With theelectric field between p+ doped layer 405 and n+ doped layer 415,electrons 427 drift from absorption region 435 into multiplicationregion 437. With the high electric field present in layer 413 ofmultiplication region 437, impact ionization occurs with the electrons427, which generates additional electron-hole pairs and thereforeresults in the multiplication or amplification of the photocurrentgenerated in absorption region 435. The holes 429 and electrons 427 arethen collected by contacts that are coupled to layers 405 and 415 for anembodiment of the present invention.

As further illustrated, light from illumination 417 that is not absorbedin the first pass through avalanche photodetector 403 is reflected fromburied oxide layer 425, illustrated as SiO₂ in FIG. 4B, and isrecirculated back and forth through avalanche photodetector 403 asshown. As a result, the light from illumination 417 is recycled withinthe absorption region 435 and multiplication region 437, therebyincreasing the probability of absorption of illumination 417 andimproving the performance of avalanche photodetector 403 in accordancewith the teachings of the present invention.

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to be limitation to the precise forms disclosed. Whilespecific embodiments of, and examples for, the invention are describedherein for illustrative purposes, various equivalent refinements andmodifications are possible, as those skilled in the relevant art willrecognize. Indeed, it is appreciated that the specific wavelengths,dimensions, materials, times, voltages, power range values, etc., areprovided for explanation purposes and that other values may also beemployed in other embodiments in accordance with the teachings of thepresent invention.

These modifications can be made to embodiments of the invention in lightof the above detailed description. The terms used in the followingclaims should not be construed to limit the invention to the specificembodiments disclosed in the specification and the claims. Rather, thescope is to be determined entirely by the following claims, which are tobe construed in accordance with established doctrines of claiminterpretation.

What is claimed is:
 1. An apparatus, comprising: a first regionincluding a first intrinsic material layer different from and directlyin contact with a second intrinsic material layer such that lightincident on a first side of the first intrinsic material layer passesthrough the first side to a second side of the first intrinsic materiallayer opposite the fist side and enters the second intrinsic materiallayer, the first intrinsic material layer to absorb light in a firstrange of wavelengths and the second intrinsic material layer to absorblight in a second range of wavelengths, wherein the second intrinsicmaterial layer only includes Germanium; a second region proximate to anddistinct from the first region, the second region including anotherintrinsic semiconductor material layer; a first doped layer disposedbetween the first region and the second region; a second doped layerdisposed adjacent to the second region on an opposite side as the firstdoped layer; a contact layer disposed within the first intrinsicmaterial layer; and doped guard rings disposed within the firstintrinsic material layer and at least partially surrounding the contactlayer to confine an electric field, wherein the light enters the firstregion incident through the contact layer between the doped guard rings,wherein photo-generated charge carriers that are generated in the firstregion and swept into the second region are multiplied in the secondregion when a voltage is applied across the contact layer and the seconddoped layer.
 2. The apparatus of claim 1, wherein: the first intrinsicmaterial layer comprises intrinsic silicon, and the other intrinsicsemiconductor material layer comprises intrinsic silicon.
 3. Theapparatus of claim 2, wherein the first doped layer comprises a P dopedlayer and the second doped layer comprises an N doped layer.
 4. Theapparatus of claim 3, wherein the contact layer comprises a P dopedlayer through which the light passes prior to being incident upon thefirst intrinsic material layer.